An alkyl glycoside is a sugar derivative surfactant which is less irritating than other surfactants. Also, though it is a nonionic surfactant, alkyl glycosides form stable foam per se, and furthermore, exert a foam-stabilizing effect on other anionic surfactants. Recently, these characteristics have made alkyl glycosides highly noteworthy.
Although alkyl glycosides as novel surfactants have the above-mentioned noteworthy characteristics, it is quite difficult to produce them in the form of a commercially useful product.
The most serious problem in the production of alkyl glycosides is that various procedures during the production process thereof frequently cause deterioration of the hue of the product.
Thus, it has been attempted to prevent the deterioration of the hue of an alkyl glycoside during the production process, and some methods therefor have been proposed. For example, JP-A-59-139397 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process wherein a higher alcohol is reacted with a monosaccharide in the presence of an acid catalyst composition comprising an acid catalyst and a reducing agent to thereby provide an alkyl glycoside; European Patent 0132043 reports a process wherein an acid form of an anionic surfactant in the form of an acid is used as a catalyst; and European Patent 0132046 discloses a process wherein neutralization with an organic base is conducted at the termination of a reaction. Furthermore, JP-A-62-192396 discloses a process wherein a viscosity depressant is added upon the separation of the formed alkyl glycoside from the unreacted and recovered alcohol, since the high viscosity and poor heat stability of the alkyl glycoside would cause the particularly serious deterioration of the hue. However, none of these known processes can provide an alkyl glycoside having satisfactory hue for commercial practice.
JP-A-47-16413 (corresponding to U.S. Pat. No. 3,565,885) describes a process wherein an alkyl glycoside reaction product is contacted with a basic anion exchange resin prior to the separation so as to provide an alkyl glycoside having excellent hue; in this case, however, there is a problem with respect to the evolution of an amine odor causing the deterioration of the alkyl glycoside reaction product with respect to odor.
Furthermore, JP-A-61-33193 (corresponding to U.S. Pat. No. 4,557,729) describes a process wherein the finally obtained alkyl glycoside product is bleached with hydrogen peroxide and sulfur dioxide. However, this process is a rather drastic measure, and it is accompanied by other problems such as the deterioration with respect to odor and poor stability upon storage.
In addition, JP-A-1-290692 (corresponding to U.S. Pat. No. 4,762,918) proposes a process for improving the color of a glycoside composition which comprises contacting a glycoside composition containing colored humin with a hydrogen source (for example, hydrogen, sodium borohydride), but the product is still deficient in hue (see Comparative Example 4 below).
Thus, there have been reported a number of methods for the production of an alkyl glycoside excellent in hue. However, each alkyl glycoside thus produced shows an unsatisfactory hue or suffers from deterioration with respect to odor. That is, none of the foregoing methods can provide an alkyl glycoside which is excellent in both hue and odor.
SUMMARY OF THE INVENTION
Under these circumstances, we have conducted extensive studies in order to produce an alkyl glycoside excellent in hue and odor. As a result, we have found that deterioration of the hue of an alkyl glycoside is caused by reducing sugars such as the unreacted glucose, the acidic component formed by the decomposition of these reducing sugars and furan derivatives contained in the alkyl glycoside reaction product. We have further found that these materials causing coloration may be readily reduced by contacting the alkyl glycoside reaction product containing these materials with a specific metal/hydrogen complex. We have further found that the separation of the alkyl glycoside after the above-mentioned treatment by, for example, distillation, does not bring about any deterioration of the hue of the alkyl glycoside, thus completing the present invention.
As indicated above, it is an object of the present invention to provide a process wherein an alkyl glycoside reaction product containing materials causing coloration is contacted with a specific metal/hydrogen complex, to thereby preliminarily remove the materials causing coloration before they are converted into color bodies upon heating.
In the process of JP-A-1-290692, on the other hand, an alkyl glycoside containing color bodies is bleached with a metal/hydrogen complex such as sodium borohydride after removing the unreacted alcohol. Accordingly, the present invention is distinguished from that of JP-A-1-29062, e.g., in that the formation of color bodies is prevented before it happens.
Thus, in accordance with the present invention, there is provided a process for the production of an alkyl glycoside excellent in hue, which comprises (1) reacting a sugar with alcohol to obtain an alkyl glycoside reaction product containing an unreacted higher alcohol, (2) reacting the resulting product obtained with a metal/hydrogen complex represented by formula (I) EQU M(BH.sub.4).sub.x (I)
wherein
M is an alkali metal, Ca, Zn, or (CH.sub.3).sub.4 N; and x is 1 when M is an alkali metal or (CH.sub.3).sub.4 N and x is 2 when M is Ca or Zn;
(3) separating the resulting reaction mixture into the alkyl glycoside and the unreacted higher alcohol, and (4) decomposing the remaining metal/hydrogen complex with an acid.